Knives date from the Stone Age and are important to the present day. Exposed blades on knives can be a hazard and thus sheaths were developed to cover the blade. As early as the Roman Empire folding knives were developed. Such folding knives are called pocket or army knives and provide a safe means of carrying and storing bladed tools.
U.S. Pat. No. 2,037,914 discloses a knife offering razor sharpness. Such knives are now known as utility, shop or trimming knives. A variety of these knives have been developed including ones with retractable blades such as U.S. Pat. No. 4,733,734, which employs a button for retracting the blade. Crosses between pocket and utility knives are known.
A scalpel is a small lightweight knife used to cut tissue during dissection or surgery. For such use it is essential to control the depth of the cut since the number of layers or thickness of tissue to be cut varies. For a surgery, it is desirable to have a blade that automatically retracts to protect health care personnel against serum transmitted diseases such as AIDS and hepatitis B. Most scalpels use a disposable blade and holder. One such holder is described in U.S. Pat. No. 4,930,220, incorporated herein by reference.
Coupon cutters, designed to cut sections from newspapers and magazines also require control of the blade depth to avoid cutting extra pages. Additionally, it is desirable that such cutters be small enough to be carried about in a pocket or purse. Hook coupon cutters place a guide between the blade and the layer that is not to be cut. Others have a very small exposure of a nonadjustable blade.
Presently, sheathed blades require a specific movement by the user such as folding, unfolding or sliding a button. Operation takes conscious effort on the part of the user and sheathing mechanisms add substantially to the cost of manufacture.
However, the inventor is unaware of a self-retracting, adjustable depth knives of reasonable cost.